1. Field of the Invention
The present invention relates to a photographic lens system, and more particularly to a 5-group, 5-element photographic lens system having an angle of view greater than 20.degree. and an aperture ratio of about F/2.0.
2. Description of the Prior Art
A Gauss type lens system and modifications thereof are used in various single lens reflex type camera system as a standard lens or a telephoto lens. Various types of such lens systems consist of six or seven lens elements.
Then, in order to decrease the number of the lens elements, 4-groups, 5-elements or 5-groups, 5-elements constructions have been suggested. For example, in Japanese Patent Publication No. 30-9465 discloses 4-groups, 5-elements lens system in which the rear lens group located on the image side consists of two elements having a negative meniscus lens element concave to the object side and a positive lens element. However, although such lens system is effective for making the construction of the lens system more simple, it is difficult to obtain a large aperture ratio reaching about F/2.0.
On the other hand, in Japanese Patent Publication No. 31-6986, 5-groups, 5-elements lens system is disclosed in which the front lens group located on the object side consists of two lens elements including a positive lens element and a negative meniscus lens element concave to the image side. Such a lens system is capable of obtaining a large aperture ratio, compared with the above 4-groups, 5-elements lens system. But it has a defect in that the coma can not be corrected well in the middle area of the image. Thus, the capability of the representation is not sufficient.
Furthermore, the other lens system of 5-groups, 5-elements type have been also suggested in which the lens system consists, from the object side, of a positive first lens element convex to the object side, a positive meniscus second lens element convex to the object side, a negative meniscus third lens element concave to the image side, a negative meniscus fourth lens element concave to the object side, and a positive fifth lens element. Such a lens system is known as a Xenotar type, and has a characteristic that the object side surface of the fourth lens element is almost concentric with the image side surface thereof. Therefore, polishing of such surfaces and centering and edging are difficult in manufacturing the fourth lens element. Then, if the fourth lens element is made of a synthetic resin material, it would be free from such difficulty. However, in the conventional design, the fourth lens element is made of high-index glass material. Thus, it has been difficult to obtain the sufficient optical performance with applying the synthetic resin material to the fourth lens element instead of the glass material.